If you would go back in time and kill your grandfather (before your parent was born), would you cease to exist? If you would, then there would be no way for you to kill your grandfather, because you don’t exist. If you don’t exist, then your granfather is still alive, and so you will exist. So if you do exist, then you can kill your grandfather.
A ???? ?????. I believe ? ????? ???? speaks about this in numerous places, about “A ???? which if it will be ?? will be ???? itself” – For example, by an ??? ????? who gets money from somebody ?? ??? ???? ??? ???? ??, the ??? can keep the money, even though generally we say ?? ??? ??? ???. R Shimon explains how this works: The money was given with a ???? that the ???? shouldn’t get it. So if the ???? gets it, then ????? the ??? never got it in the first place; therefore the ???? can’t get it either. So if the ???? recieves the money, a direct cause of that will be that the ???? loses the money. So it is a ???? which if it would be ?? would cancel itself out, rather than it being ?? and then being ???? itself, we say it isn’t ?? in the first place.
Same sort of ????